CONTEXT: The recent Third International Workshop on the Management of Asymptomatic Primary Hyperparathyroidism (PHPT) set 60 ml/min as the precise level of glomerular filtration rate (GFR) below which surgery is recommended because it is considered a threshold of concern in patients with PHPT. OBJECTIVE: The aim of the study was to investigate the relationship between different stages of renal insufficiency and PTH levels in PHPT patients. DESIGN: We conducted a cross-sectional study. PATIENTS AND METHODS: We studied 294 consecutive PHPT patients. Biochemical evaluation included total and ionized serum calcium, phosphate, creatinine, immunoreactive intact PTH, and 25-hydroxyvitamin D3 levels in the fasting state. GFR was assessed with the Modification of Diet in Renal Disease Study formula. RESULTS: The mean GFR of the whole group of PHPT patients was 92.3 +/- 31.6 ml/min x 1.73 m(2). The patients were divided into four groups according to National Kidney Foundation Disease Outcomes Quality Initiative (K/DOQI) guidelines: group 1 with normal or increased GRF (>90 ml/min x 1.73 m(2); n = 153); group 2 with mild decreased GFR (60-89 ml/min x 1.73 m(2); n = 90); group 3 with moderately decreased GFR (30-59 ml/min x 1.73 m(2); n = 45); and group 4 with severely decreased GFR (<30 ml/min x 1.73 m(2); n = 6). PTH levels were comparable across groups 1-3, whereas group 4 showed significantly higher PTH levels (P < 0.0001). CONCLUSION: In our series of PHPT patients, only a severe impairment of GFR was characterized by a further PTH increase. These findings challenge the concept of a PTH elevation below the threshold of 60 ml/min of GFR.